The present invention relates to a fiber optic current sensor in accordance with the preamble of patent claim 1.
A fiber optic current sensor of the generic type is known from EP-A-0,856,737. It has a magneto-optically active sensor fiber which is wound in a coil and encloses a current conductor. The sensor fiber is connected, at least at one end, via a phase delay element to a further optical fiber, a so-called feed or return fiber, via which light can be coupled into or out of the sensor fiber. The sensor fiber is typically fabricated from quartz glass and has a round core cross section. Circularly polarized light propagates in the sensor fiber. The feed and return fibers preferably have an elliptical core cross section and propagate linearly polarized light. Acting as phase delay element is a birefringent fiber segment which is arranged between the sensor fiber and feed fiber. The fiber segment has main optical axes which are aligned at 45xc2x0 to the main axes of the feed and return fibers. Its length is usually selected such that it acts as a xcex/4 phase delay element. It thereby converts linearly polarized light of the feed and return fibers into circularly polarized light which propagates in the sensor fiber.
The sensor fiber is operated either as a Sagnac interferometer or, when one of its ends is silvered, as reflection interferometer. In both cases, two circularly polarized optical waves propagate in the sensor fiber. Here, the waves are oppositely directed in the case of the Sagnac interferometer, and run in the same direction in the case of the reflection interferometer. Both waves have the same sense of polarization in the Sagnac interferometer, being either left-circularly or right-circularly polarized. They have an opposite sense of polarization in the reflection interferometer.
If current is flowing through the current conductor, the current generates a magnetic field which leads to a differential phase shift between these two oppositely directed or identically directed optical waves. This effect is termed the magneto-optic or Faraday effect. The phase shift produced is proportional in this case to the current, and amounts in the Sagnac configuration to
xcex94"PHgr"S=2V N Ixe2x80x83xe2x80x83(1)
and in the reflection configuration to
xcex94"PHgr"R=4V N Ixe2x80x83xe2x80x83(2)
V denoting the Verdet""s constant of the fiber, N the number of fiber windings in the coil, and I the current intensity.
The sensor fiber described in EP-A-0,856,737 is certainly free from mechanical stresses, and so the sensor signal obtained is not disturbed by a temperature-dependent, stress-induced linear birefringence. However, the Verdet""s constant V likewise exhibits a temperature dependence which is noticeable even in the case of an ideal, stress-free fiber coil. Thus in the case of a quartz fiber the temperature dependence per xc2x0 C. is approximately 0.7xc3x9710xe2x88x924. The sensor signal therefore changes by 0.7% given the temperature fluctuation by 100xc2x0 C. However, for applications of the current sensor in high-voltage installations there is frequently a need for substantially higher accuracy of current measurement. Typical values are xc2x10.2% in the temperature range from xe2x88x9240xc2x0 C. to +85xc2x0 C.
It is therefore the object of the invention to create a fiber optic current sensor of the type mentioned at the beginning which exhibits an improved temperature compensation.
This object is achieved by a current sensor having the features of patent claim 1.
According to the invention, the current sensor has at least one phase delay element with a temperature dependence which at least approximately compensates for a temperature dependence of a Verdet""s constant of a sensor fiber.
The compensation comes about by virtue of the fact that the phase delay element has an appropriate phase delay angle which deviates from a phase delay angle of an ideal phase delay element. In this case, it is selected to be larger or smaller as a function of a sign of its temperature dependence than that of the ideal phase delay element, for example as a 90xc2x0 phase delay angle of an ideal xcex/4 segment.
Moreover, in the case of a Sagnac interferometer, the phase delay angle is selected as a function of a relative alignment of fast axes of the phase delay element to one another.
The phase delay element is preferably a birefringent fiber segment with an elliptical core cross section, it being possible to set the phase delay angle by selecting the length of the segment.
Further advantageous embodiments follow from the dependent patent claims.